fbrems, yes, the cause of the cracks is often initially due to the failure of any number of cooling system components and the high combustion temp. But, if one of these blocks is ever aloud to overheat, cracks are soon to follow. Cracked blocks are Very common- 80% of 4.0, 25% of 3.9 and 4.2 (as stated in des Hamel's, how to power tune rover v8 engines). The higher failure of the 4.0 relates to the 196 f thermostat in the HSE used to keep the engine temp high (clean burning). Builders like Wildcat and JE engineering have gone to great lengths to compensate for these flaws in order to produce their race engines.
Lets not be too romantic about these trucks. I think they are the most capable (civilian) off- road trucks on the planet. But, since when are rovers known to be reliable. Sure they can be driven for ever, if maintained. Howeer, i dont think there is a single part on a disco designed to last more that 50K miles (joke, kind of).
Once the coolant level is below the hose to the overflow, it cant push any more out. But, by this time you will hear coolant surging through the heating system as there is air (combustion) trapped in it. When the engine is off, the pressure either leaves through the unobstructed rad cap or the other way out. When the problem gets worse and you are told by a mechanic or dealer that the head gaskets are bad, check the rear two cylinders to see if they have been steam cleaned. This is caused when cracks lead to the reverse- coolant blowing by the sleeves into the bore. You can buy a dye test for this. Ask them at NAPA. Even a good test kit is cheaper than a new rad.
i could be totally wrong about your particular problem. But I would not admit that throwing money at a problem is preferred to performing actual diagnostic tests. Moreover, these tests can be done by anyone in a few minutes for little cash.
